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My friend's iphone charger exploded. She was charging an iPad with it. Since the iPhone power supply has a current rating of 1A while the iPad charges with a 2A current, many are blaming her that she didn't use the iPad charger since the iPad drew more current than the rated of the supply thus causing it to overheat and explode.

As far as I know the current rating of the charger used is simply the maximum current that it can deliver to the device. If the device can draw 2A, fine but it doesn't mean it needs to draw 2A by force. If the charger can only supply 1A than it is going to charge the device with 1A which obviously takes slower for it to charge than if the charger could provide 2A.

What is the correct reasoning please?

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    \$\begingroup\$ If the charger is really cheap and doesn't have sufficient (or any) self-protection circutry, then the iPhone might very well attempt to pull more current than the charger is safely able to provide. It probably did provide more current than it's rated to provide, got very hot in trying so hard, and eventually melted or broke something. You can easily over-work an electronic device that doesn't have extra circuitry for self-protection. More expensive devices will include some (some more, some less) self-protection circuits for common faults and avoid damage. But probably not the case here. \$\endgroup\$ – jonk Dec 23 '19 at 1:44
  • \$\begingroup\$ It was an original iPhone charger which came in the box! \$\endgroup\$ – Simon Dec 23 '19 at 1:48
  • \$\begingroup\$ Wow! So, from Apple? Just using a lower rated Apple product on a device that expects more current? I'd be pretty unhappy with them, then. In fact, I'll be pretty unhappy with them, right now. I'd expect better. Apple can afford to do their due diligence for their power supplies. They make enough money. \$\endgroup\$ – jonk Dec 23 '19 at 1:50
  • \$\begingroup\$ Definitely. I'll try to post the photos \$\endgroup\$ – Simon Dec 23 '19 at 1:52
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    \$\begingroup\$ Apple products are notorious for being designed to only charge when the "correct" charger is used. If it was a genuine product and the charger worked at all it should be acceptable to use it. || A failure of this nature suggests a rather hard short across the mains. Even a low quality charger would not usually fail in this manner. I'd return it to the reseller and maintain a polite but highly robust attitude re obtaining a replacement. Regulatory authorities may be interested in a failure of this sort - and 'Apple' should show great interest in exchanging it for a free replacement. \$\endgroup\$ – Russell McMahon Dec 23 '19 at 2:48
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In general your thinking is correct. Usually charging with an underrated charger should not be a problem, but may result in slow or no charging of the device.

If her other charger was a cheap charger that was not short-circuit protected it could potentially have overheated, and thus, failed.

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    \$\begingroup\$ Overloading a power source can easily overheat it. \$\endgroup\$ – Bort Dec 23 '19 at 1:18
  • \$\begingroup\$ Could it be that the charging circuitry of the device would limit the current demand from the supply if for example it detects a drop in voltage from the power supply due to over-current? \$\endgroup\$ – Simon Dec 23 '19 at 1:22
  • \$\begingroup\$ @Simon Some power supplies will turn themselves off, if and over-current (short-circuit) is detected. \$\endgroup\$ – theeren Dec 23 '19 at 2:24
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The supply is responsible for protecting itself.
The load is responsible for regulating it's charge current.

If the demand is greater than the supply, the voltage will drop and thus demand will drop. It may stop and burp restart if intermittent but it should not blow up.

Judging by the carbon footprint there appears to be a fault not due to the user.

If USB charger does not follow these design rules, avoid rebuying them.

If it was from a major brand. Return to supplier and ask for a replacement and explain the incident.

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